Conventionally, there has been known one in which, when a load system driven by a motor has been operated by a specified target amount, the drive system is automatically stopped. For example, in the case of a power window of an automobile, its motor is driven with a switch operation by the crew to move the window glass downward or upward, and upon achieving full open or full closure, a limit switch or a pressure-sensitive sensor operates to automatically stop the motor. Also, in the case of an elevator, an automatic door, a shutter or the like, it is normal to stop its drive system by position control with a limit switch or the like as described above.
In the aforementioned control device for motor driving system, the drive system stops at the point of time when the load system reaches a specified target position. Therefore, the drive system does not stop even if any error occurs until it reaches the target position. For the above reasons, a variety of irreparable troubles occur sometimes. For example, in the case of the aforementioned power window, there has been the danger that a human hand or neck may be caught in the opening or closing window glass to get injured.
Thus, the prior art apparatus detects the achievement of its purpose of moving the load system by a specified amount by means of a limit switch or a pressure-sensitive sensor and feeds this back to the motor to stop it. Therefore, the system has been unable to cope with the aforementioned error occurring in the course of achieving the purpose. Accordingly, there has been taken the measure of providing a torque limiter or the like between the motor and the load or detecting the error by reading a change of current or phase of a motor power source. However, they are easily influenced by the fluctuation of the power source or temperature, and therefore, no correct control can be expected.
A position detecting means of the limit switch or the pressure-sensitive sensor requires a stationary fixed member in order to fix the detection means. For example, in the case of the power window, the limit switch and the pressure-sensitive sensor are mounted to the window frame. However, in an automobile having no window frame, it is difficult to provide a position detecting means such as the limit switch or the pressure-sensitive sensor. In such a case, it has been executed to drive the motor by means of an inverter and stop the motor by detecting a torque increase as a consequence of the change of its input current or to stop the motor by detecting a torque increase as a consequence of the distortion of an output shaft of the motor. However, such an apparatus generally adopts the method of measuring the change of an optical signal from a stationary system having the power source in order to measure the distortion of the rotating shaft, or the method of measuring the change of a resistance value of a strain gauge by means of a slip ring. However, either method has an unstable factor and still contains a problem in achieving correct control.
Representative motors that have been conventionally used are the direct current motor, the synchronous motor and the induction motor. The direct current motor requires maintenance since it has a mechanical commutator, and it has the disadvantages of: its voltage being not increasable; a sharp change of current being unable to be followed; its capacity being limited; and sparks being generated. In view of the above, the synchronous motor which has no commutator and of which current is controlled based on a position detection signal of its rotor has been put into practical use. However, it requires a slip ring or a rotary transformer in order to supply an excitation power to the rotor in the case of a large capacity. In contrast to this, the induction motor has a simple structure and solidity and is inexpensive. In order to freely control the induction motor similarly to the direct current motor, a vector control has been put into practice. However, complicated calculation is required for the vector control, and a stable power source is necessary. In such a conventional motor, there have been the control factors of current, voltage, frequency, angle of rotation and rotational frequency. These control factors exert influence on the fluctuation of the power source and the change of the rotational frequency, and this causes a problem that high-accuracy control cannot be executed.